Recent miniaturization of the wirings in ULSIs disadvantageously contributes to a reduction in reliability of the wirings. In particular, due to a development in multilamination of wirings, there is increasingly used a so-called stacked structure in which upper and lower plugs made of a high-melting-point metal such as W (tungsten), TiN or the like and its compound, are respectively disposed, as opposite to each other, on and under a wiring. In such a stacked structure, that portion of the wiring held by and between the plugs is dis-advantageously weak in reliability.
To enhance the wiring reliability, a first method of prior art is for example arranged to prevent the stress migration that stress from a protective layer disconnects wirings. More specifically, a wiring layer large in grain size is formed in a region in which a major portion of the electric current flows, and a wiring layer small in grain size is formed at a position to which stress is relatively readily applied, such as a surrounding of the wiring (in particular, lateral wall or upper end portion) (See Japanese Patent Laid-Open Publication No. 5-275426).
According to a second method of prior art, there is proposed a stacked structure in which the width of each wiring is smaller than the width of the lower-layer-side connection hole and in which the bottom of the upper-layer-side connection hole extends into the lower-layer-side connection hole such that the wiring is reinforced as if entirely surrounded (See Japanese Patent Laid-Open Publication No. 8-167609).
According to the first method of prior art, however, the wiring is formed by forming a wiring layer small in grain size at a lateral wall of a wiring layer large in grain size. This excessively increases the width of the wiring and is therefore not suitable for miniaturization.
According to the second method of prior art, the width of each of the upper- and lower-layer-side connection holes is inevitably larger than the width of the wiring. This is neither suitable for miniaturization. Further, no consideration has been taken for stress from a high-melting point metal or its compound which surrounds the wiring.